Current interlaced video streams may carry video signals mastered from progressive frames. For example, 24 Hz progressive film may be converted to PAL (Phase Alternating Lines) interlaced video by creating two fields from each progressive frame. This is referred to as a 2:2 cadence. Similarly, 24 Hz progressive film may be converted to NTSC (National Television System Committee) interlaced video by creating alternately three fields and two fields from the progressive frame. This is referred to as a 3:2 cadence. Other cadences include 5:5 or 4:4 for 12 Hz animation and 3:2:2 for sped up film. These video streams are later referred to as film-based sources. In contrast, video cameras typically capture each field at a separate moment in time. In this case, the video stream is not mastered from progressive frames and no consistent cadence is present. This is later referred to as video-based sources. Sporting events or other live television broadcasts typically are video-based sources.
Displaying such video streams on progressive video systems involves de-interlacing techniques. A device designated for performing a de-interlacing technique is often referred to as a de-interlacer or de-interlacing unit. De-interlacing is a process of converting interlaced fields into progressive video frames by doubling the number of lines. For example, current de-interlacers may use field jam, line interpolation, or a combination of the two methods to create the new lines. Field jam combines two consecutive fields (an even field and an odd field), doubling the vertical resolution. Field jam may introduce artifacts when there is motion between the two fields, so it is often used for areas with little motion. Line interpolation interpolates the new lines from a single field and is often used for areas with motion between fields. De-interlacers may exclusively use field jam for video mastered from progressive frames to reconstruct the original source. This is referred to as film pulldown, or sometimes 3:2 or 2:2 pulldown.
As incoming video streams may or may not carry video mastered from progressive frames, accurate detection of cadences becomes important to instruct the de-interlacer so that it may optimally de-interlace film-based sources without introducing artifacts in video-based sources. For example, misdetection of a video-based source as a film-based source or misdetection of the position in the cadence may cause combing artifacts and/or reduced frame rate.